I can only comment on what I do (with microBGAs: 0.8mm pitch).
I tent everything that's not used as a testpoint. Most particularly,
at the pitch of BGA I'm using, all vias under the BGA are tented.
With the fine pitch BGA, an untented via results in a soldermask
opening that intersects with that of the adjacent BGA land. If I
tried to place a BGA on the resulting footprint, there is a high
probability of the ball collapse during reflow resulting in a
short.
I just scrapped a prototype run of 60 boards because I forgot to
go back and double check that all vias under the BGA were tented.
With larger pitch BGAs it may be that you can maintain soldermask
design rules even with untented vias. If so, I don't think there's
any absolute necessity to tent the via.
If you're using liquid photoimagable soldermask then the tenting
won't be perfect - the soldermask covers the surface but usually
leaves the via hole open. If you want to ensure complete tenting,
you'll need to specify dry film soldermask. The downside with
this is that it is much thicker than LPI, which may have flow-on
effects in assembly and reliability. e.g. If you need to use
a thin paste screen (because of fine pitch parts), the standard
solder volume deposited for parts like 0402s may be insufficient
because the component will be elevated from the copper by the
thickness of the soldermask. This sort of thing is an issue you
should discuss with your assembler. Long live DFM :-)
As mentioned by others, Protel allows you to tent vias simply
by checking the "tented" check box on the via properties pane.
Cheers,
John Haddy
> -----Original Message-----
> From: Bagotronix Tech Support [mailto:[EMAIL PROTECTED]]
> Sent: Thursday, 23 August 2001 4:36 AM
> To: Protel EDA Forum
> Subject: [PEDA] tenting vias
>
>
> Hello, all:
>
> I am designing a board with a 388 BGA chip. I have questions about vias:
>
> 1) Should I tent the vias under the BGA?
> 2) If (1) is yes, should I tent other vias on the board?
> 3) If (1) is yes, what substance/method is used to tent vias?
> 4) If (1) is yes, what do I tell the PCB fab house to get them
> to tent the
> vias? Is it as simple as "tent the vias"?
> 5) If blind vias are used, do they need tenting also?
>
> The BGA has a 1.27mm ball grid. I am considering a 25 mil pad,
> 12 mil via,
> 31 mil solder mask opening diameter. Does this sound good? If not, what
> would you recommend?
>
> Best regards,
> Ivan Baggett
> Bagotronix Inc.
> website: www.bagotronix.com
>
>
>
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* To post a message: mailto:[EMAIL PROTECTED]
*
* To leave this list visit:
* http://www.techservinc.com/protelusers/leave.html
* - or email -
* mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum
*
* Contact the list manager:
* mailto:[EMAIL PROTECTED]
*
* Browse or Search previous postings:
* http://www.mail-archive.com/proteledaforum@techservinc.com
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
